Alessandra Venuti

We present paleomagnetic and mineral magnetic results from ocean sediments from the southern Okinawa Trough (west Pacific). We ob- tained samples from two holes from Ocean Drilling Program Site 1202 and determined the natural remanent magnetization, magnetic suscep- tibility, anhysteretic remanent magnetization (ARM), hysteresis proper- ties, and thermomagnetic behavior. Hole 1202A was studied between 100 and 120 meters below seafloor (mbsf) and Hole 1202B between 0 and 140 mbsf, both at 1-cm resolution. Hysteresis properties and ther- momagnetic behavior were measured on selected samples. The mea- surements show a stable magnetization carried by pseudo-single- domain-sized low-titanium magnetite. Magnetic inclinations are pre- dominantly positive and record the Brunhes (C1n) normal polarity chron. Susceptibility and ARM, as well as the environmentally signifi- cant rock magnetic ratios (ARM/k and ARM30 mT/ARM0 mT), reflect changes in sediment input from Taiwan and the East China Se...

The objective of this study is to advance our understanding of the late Cenozoic environmental and paleoceanographic history of the Southern Okinawa Trough and the Kuroshio Current. Our investigation is based on the environmental magnetic record of a suite of u-channel samples that span the upper 140 m (0.42 m.y.) of ODP Site 1202. This site was drilled in the southernmost Okinawa Trough off the northeast coast of Taiwan and provides, with sedimentation rates of up to 9 m/k.y., one of the highest-resolution records of environmental and paleoceanographic change ever recovered. Variations in the concentration of fine-grained magnetite (anhysteretic remanent magnetization, ARM), magnetic grain size (ARM/k), magnetic hardness (ARM(30)/ARM(0)), and overall mineralogy (susceptibility, k) reflect fluctuations in climate, sea level, source material, and depositional environment. Eleven AMS C-14 dates of planktonic foraminifers and scaphopod samples combined with oxygen isotope stratigraphy (Wei et al., 2005) provide a relatively low- resolution, but well-defined age model. Based on the magnetic characteristics we distinguished five distinctive intervals with boundaries at 11.3, 14.3, 18.6, and 27.4 ka. From the beginning of the record at about 40 ka to 27.4 ka (MIS-3) all magnetic parameters remain remarkably constant indicating very little variation in sediment source and processes. Sea level during this time interval fluctuated at around -80 m. At 27.4 ka a significant decrease in magnetic grain size is accompanied by an increase in the amount of magnetic material. This change coincides with a drop in sea level below -80 m in the buildup to the sea level lowstand of the Last Glacial Maximum (LGM). Sediment source during this time interval was predominantly the East China Sea (Wei, 2006; Diekmann et al., in press). MIS-2 is characterized by constantly increasing magnetic grain size and a decreasing amount of magnetic material. At 14.3 ka after melt water pulse (MWP) 1A, which is expressed as a short spike in all magnetic parameters, the magnetic signature changes significantly, shows very little variability and remains constant until MWP-1B at 11.3 ka. An effect of the Younger Dryas is possibly preserved in the magnetic susceptibility signal. Associated with a significant sea level rise at the onset of the Holocene, a change in sediment source to Taiwan (Wei, 2006; Diekmann et al., in press) and the onset of the Kuroshio Current as sea level rose above -50 m, magnetic parameters shift and remain relatively constant throughout the Holocene. This allowed for the extraction of a high-resolution relative paleointensity record from 0-9.4 ka. MWP-1C is associated with a short and sudden increase in magnetic grain size.

Site 1202 was drilled during ODP Leg 195 in the southernmost Okinawa Trough off the east coast of Taiwan. The southern Okinawa Trough currently has a high rate of sedimentation due to the enormous input of terrigenous material from the East China shelf and Taiwan. The 410 m composite section recovered at Site 1202 consists of rapidly-deposited calcareous silty clay, and based on biostratigraphic evidence, the entire section is believed to be younger than 127,000 years old. Thus, the recovered sediment represents a unique opportunity to determine environmental change associated with the Kuroshio Current as well as the evolution of the Earth's magnetic field on centennial to multi-decadal time scales. We have been studying the paleomagnetic and environmental magnetic record of a suite of u-channel samples that span the upper 140 meters of the site. The sampled section appears to be yielding a reliable magnetic record although both stability and magnetic intensity decrease downcore. The NRM/ARM record from the uppermost 40 m has features that can been tentatively correlated with the Holocene paleointensity record from Fish Lake, Oregon. This suggests that Site 1202 is indeed one of the highest resolution records of the geomagnetic field and environmental change yet found.

We investigated u-channels from the top 36 meters of Hole 1202B collected in the Southern Okinawa Trough during Ocean Drilling Program Leg 195. Detailed rock magnetic data demonstrate that the sediments preserve a high resolution record of paleosecular variation and a paleointensity signal spanning almost the entire Holocene. The sediments consist of homogenous, slightly calcareous, bioturbated clayey silt with isolated sandy intervals and fine sand laminae. An age-depth model was established through C-14 accelerator mass spectrometry dating of planktonic foraminfera. The studied section spans almost the entire Holocene (0-9.4 kyr) and exhibits sedimentation rates close to 400 cm/kyr. The magnetic properties are dominated by stable, pseudo-single domain low-titanium magnetite with a Curie temperature of about 540 deg C. High-field hysteresis data and the grain-size sensitive ratio of anhysteretic remanent magnetization (ARM) to low field magnetic susceptibility indicate a narrow range of grain sizes and concentrations. Key magnetic parameters vary by less than a factor of four, thereby fulfilling the criteria for relative paleointensity determinations. The relative paleointensity was extracted by normalizing the intensity of the natural remanent magnetization (NRM) by the ARM and by the low field magnetic susceptibility. Both normalizations yield very similar results. Spectral analysis indicates that the record is not significantly affected by local environmental conditions. Comparison of this Western Pacific paleointensity curve with other curves suggests a geomagnetic origin for the observed variations. Millennial-scale features in our record correlate with variations of the archeomagnetic dipole moment, which implies that the sediment from Hole 1202B recorded changes of the geomagnetic field over the investigated time interval.

We have been studying the paleomagnetic and environmental magnetic record of a suite of u-channel samples that span the upper 140 meters of ODP Site 1202. This site was drilled during Leg 195 in the southernmost Okinawa Trough off the east coast of Taiwan. The southern Okinawa Trough currently has a high rate of sedimentation due to the enormous input of terrigenous material from the East China shelf and Taiwan. The sampled section yields a reliable paleomagnetic record and an environmental magnetic record that has five distinct zones. Analysis of hysteresis parameters and anhysteretic remanent magnetization behavior indicate that the sediments satisfy the requirements for paleointensity determination and that either susceptibility or ARM can be used as normalization parameters. Using either method, we can correlate the upper 50 meters of Site 1202 with the Holocene paleointensity record from Fish Lake, Oregon. The remaining 90 meters can be correlated with the NAPIS-75 paleointensity stack. These correlations show that the base of the studied section is only 50,000 years old and that Site 1202 represents one of the highest resolution records of geomagnetic field behavior and environmental change ever reported.

The Auckland volcanic field contains several maars that formed after the last interglacial and subsequently filled with sediment. Two of these maars, Pukaki and Onepoto, were recently cored as part of the Auckland Maar Lakes Project. The tephra stratigraphy of the cores indicates that sediment accumulated relatively slowly in both maars until the Holocene when ocean waters breached the craters and they filled up quite rapidly. Using u-channels, we collected 23 meters of pre-Holocene lacustrine sediment from the Pukaki core and 15 meters from the Onepoto core. Measurements of NRM, ARM and IRM were done using the long-core cryogenic magnetometer at UC-Davis. Measurements of magnetic susceptibility were made on the whole cores using a Bartington core logging sensor and on the u-channels using a Bartington field surface probe. The NRM of all samples was strong, and generally showed uni-vectorial decay during alternating demagnetization. Generally, inclinations were negative, which is to be expected for a site in the southern hemisphere. However many of the declinations were to the south in the coordinate system of the u-channels. Such a result is anomalous and suggests that despite the inclination values, the sediment in many of the core segments has been remagnetized, possibly by a drilling-induced remanence or more probably by some aspect of the core-splitting procedure. On the other hand, both cores appear to have an environmental magnetic record that can provide insights about climate change in New Zealand as well as the eruptive history of the Auckland volcanic field. For example, local basaltic tephra layers visible in the cores show up as spikes in the magnetic susceptibility record, suggesting that other spikes represent tephra layers that are not as easily discerned.

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